The present invention relates generally to the lap grinding of surfaces and specifically to a rotary lapping system which applies a controllable pressure gradient to workpieces.
Future generations of orbiting telescopes used for astronomy, information collection and transmission, and power transmission will require very large segmented mirrors. The telescope designs will necessarily make use of highly aspheric mirrors in order to keep the telescope dimensions and weight to a minimum and very thin glass sections which will limit the usable grinding and polishing pressures.
Traditional optical fabrication techniques and rotary lapping systems will be impractical for these mirrors. A review of optical surfacing techniques indicates that two-thirds of the manufacturing time is spent polishing out subsurface damage from grinding. The lightweight optical elements and mirrors of orbiting telescopes will be extremely thin (e.g., 3 mm) and aspheric with changing surface curvature.
The need to process large sections of thin aspheric sections of optical systems in a rapid and efficient manner, was recognized in two inventions of Mr. Roger Kenneth Lee. The first invention is entitled "Very High Speed Lap With Negative Lift Effect", U.S. Ser. No. 06/720,936 filed Apr. 8, 1985, the disclosure of which is incorporated by reference. In this first invention, Mr. Lee disclosed a particular mounting of the grinding pads which caused the leading edge pads to possess a negative angle of attack when rotated in an abrasive slurry liquid. The hydrodynamic effect of the negative angle of attack in the abrasive slurry liquid while adjacent to a workpiece resulted in a negative lift or suction between the grinding pads and the workpiece. The use of negative lift in rotary lap grinding allows a generation of strong cutting forces for grinding and polishing with little or no downward pressure on the workpiece. This is ideal for grinding and polishing thin section of optical mirrors.
The second invention of Mr. Lee is entitled "Very High Speed Lap With Positive Lift Effect", U.S. Ser. No. 06/720,937 filed Apr. 8, 1985, the disclosure of which is incorporated by reference. In this second invention, Mr. Lee disclosed a particular mounting of the grinding pads which caused the leading edge pads to possess a positive angle of attack when rotated in an abrasive slurry liquid. The hydrodynamic effect of the positive angle of attack in the abrasive slurry liquid while adjacent to a workpiece resulted in a positive lift on the grinding pads. The use of positive lift in rotary lap grinding helps prevent the grinding tool from digging into the surface of a workpiece slurry very high speed lap grinding.
The lapping systems, described above, depend upon the hydrodynamic forces generated during rotation in an abrasive slurry liquid to produce the desired positive and negative angles of attack in grinding pads which are flexibly mounted to a grinding tool and have a neutral angle of attack. Since the positive and negative angles of attack are produced by the configuration of the flexible mounting around the grinding pads, the positive lift and negative lift grinding tools in the prior art are separate tools: one providing positive lift through the act of rotating in an abrasive slurry liquid, and the other providing negative lift.
The need to rapidly and efficiently process large sections of optical systems suggests that highly automated methods are needed. Additionally, the grinding and polishing of aspheric surfaces efficiently entails applying a varying pressure gradient to the workpiece. In view of the foregoing discussion, it is apparent that there currently exists the need for a single rotary lapping system that can apply changing pressure gradients on workpieces, including positive and negative lift, to efficiently grind and polish thin, aspheric workpieces. The present invention is directed towards satisfying that need.